A pressurized-water reactor steam generator typically comprises a vertical housing having a feed-water inlet and a steam outlet, the lower portion of the housing being closed by a horizontal tube sheet in which the vertical legs of an inverted U-shaped heat-exchanger tube bundle are mounted. A hemispherical chamber or primary header below the tube sheet provides inlet and outlet manifolds for the inlet and outlet legs of the tube bundle, and via inlet and outlet connections places the tube bundle in circuit with the main coolant loop of the reactor pressure vessel containing the reactor core. The pressurized-water coolant goes through the primary header and the tube bundle from and to the reactor pressure vessel. Water, introduced via the feed-water inlet, rises around the tube bundle and produces steam which leaves via the generator housing's outlet, representing useful power.
The reactor core is made of assemblies of fuel rods. These rods comprise metal casings containing the nuclear fuel, and if the casing of any one of the rods leaks even slightly, the pressurized-water coolant becomes radioactive, and since the coolant is the primary medium circulated through the steam generator, the latter's primary header becomes radioactive during a normal run of the reactor. Such a condition is to be expected even though the pressurized-water coolant is continuously treated via the usual reactor auxiliary system.
As long as the heat-exchanger tube bundle of the steam generator is free from leaks, there is a complete separation of the pressurized-water coolant which circulates through the reactor core, and the feed water and output steam, so that the use of the steam as power is free from any radiation hazards.
To assure the integrity of the tube bundle of the steam generator, its tubes are inspected regularly, and if a defective tube in the tube bundle is found, the tube is put out of service by an explosive plug. The tube inspection device, such as an ultrasonic probe, for example, and if necessary, the tool which comprises ordinarily an explosively expanded plug, are attached to the end of a carrier in the form of a long flexible rod or tube or hose, which may be made of rubber, for example. With the device to be used on the end of the carrier, it is inserted in the tube to be inspected or repaired, the carrier being used to push the device in use up through the tube involved.
Both the inlet and outlet manifold portions of the primary header are provided with manholes which, when opened, permit a workman to enter and insert the device to be used on the end of the carrier, the carrier then being pushed and pulled through the tube involved as required. This possibly subjects the workman to a radiation hazard which it is very desirable to avoid.